The present invention relates to a method for forming a patterned photoresist layer on a substrate or, more particularly, to a method for forming a patterned photoresist layer on a substrate in which a layer of the photoresist material is subjected to irradiation in a pattern with actinic rays followed by dry-process developing by exposure to an atmosphere of plasma gas.
As is well known, the manufacture of semiconductor devices in the semiconductor industry is performed in a number of processing steps, typically including photolithography. The photolithography step is usually performed by use of a photoresist material which is a type of photosensitive composition. In a typical process of photolighography, a silicon wafer is first provided with a thin undercoating film of aluminum, silicon oxide, silicon nitride, polysilicon and similar material having a thickness of several hundreds of nanometers and then with a coating layer of a photoresist composition. The photoresist layer is then irradiated in a pattern with actinic rays such as ultraviolet light through a photomask having a desired pattern, followed by development with a suitable developer liquid or solution to give a patterned layer of photoresist. This patterned layer serves as a protective mask for the thin undercoating film in a subsequent etching step whereby the thin undercoating film in the unprotected areas is removed, exposing the substrate surface. Finally, the patterned photoresist layer is removed to expose the undercoating which had not been etched and remains in a pattern corresponding to the photomask pattern.
The above-described process of photolithography includes as an essential step development by a wet process using a developer liquid. A problem in wet process development is that the film of photoresist in the areas where the photoresist is not to be removed absorbs the developer liquid and becomes swollen so that the method is not quite satisfactory or practical when the pattern of the photoresist layer formed on the substrate is extremely fine. In addition, the developer liquid which usually is or contains an organic solvent causes serious problems related not only to workers' health but also to environmental pollution.
Accordingly, a recent trend has been to replace the wet-process development which has the inherent problems and disadvantages described above with a so-called dry-process development. In dry process development the photoresist layer on the substrate surface is first irradiated in a pattern with actinic rays and, then, subjected to a treatment so as to produce a difference between the irradiated and unnirradiated areas of the photoresist layer in its resistance against exposure to an atmosphere of plasma gas. The effect of the plasma gas is to ash and remove the photoresist layer in the areas where the photoresist layer has less resistance against the plasma gas, leaving a patterned layer of the photoresist composition on the substrate surface. Because no developer liquid is used at all, dry-process development is very advantageous when a very fine pattern of the photoresist layer is desired. In addition, problems associated with workers' health and environmental pollution are avoided, so that this method is highly valued in the modern semiconductor industry.
A typical procedure is reported in Japanese Patent Kokai 58-60537 for patterning a resist layer by a dry-process development according to which the resist layer is irradiated in a pattern with actinic rays, and then subjected to heat treatment at 80.degree. to 180.degree. C. for at least 5 minutes, followed by development by exposure to an atmosphere of plasma gas. In this method, however, it is not possible to achieve a sufficiently large difference in the resistance against the plasma atmosphere, between the areas irradiated and unirradiated with the actinic rays. Based upon this the residual film thickness or the residual film ratio in the patterned resist layer is small, and the cross section of a linewise patterned resist layer is trapezoidal with a broader bottom. Therefore, the dimensional accuracy in the patterning of the resist layer is relatively poor and the method is not always satisfactory when high fidelity is desired in the reproduction of a very fine pattern.